1. Field of the Invention
The present invention relates to a dielectric filter for selectively filtering a high-frequency signal having a desired frequency mainly used in a base station for a mobile communication system such as car telephones and portable telephones. More particularly, the present invention relates to a dielectric notch filter. The present invention also relates to a dielectric resonator constituting the dielectric filter.
2. Description of the Related Art
In recent years, as the development of the mobile communication system such as car telephones, a notch filter using a dielectric resonator is increasingly demanded.
Hereinafter, an exemplary conventional dielectric notch filter will be described with reference to the following figures. FIGS. 24A and 24B are external views of a conventional dielectric notch filter. FIG. 24A is a top view and FIG. 24B is a side view. In these figures, the dielectric notch filter includes cylindrical metal cavities 2401, a base member 2402, tuning members 2403, and input/output terminals 2404. The notch filter shown in FIG. 24 has five resonators. A transmission line is formed in the base member 2402 and electromagnetically coupled with the respective dielectric resonators, so as to constitute the notch filter. FIG. 25 shows the inside of a dielectric resonator used in the conventional dielectric notch filter shown in FIG. 24 in a simplified manner. In the metal cavity 2401, a dielectric block 2501 and a coupling loop 2502 for electromagnetic coupling are provided. FIG. 26 is a cross-sectional view showing an adjusting mechanism for adjusting the degree of electromagnetic coupling in the conventional dielectric resonator. As shown in FIG. 26, the adjusting mechanism includes a supporting member 2 for supporting the dielectric block 2501, a loop 4a of the coupling loop 2502, a ground part 4b of the coupling loop 2502, a handle 4c for rotating the whole coupling loop 2502, and a pole 5 of the coupling loop 2502. The pole 5 is composed of a center conductor 5a and an insulator 5b. The base member 2402 includes a transmission line 7 serving as an inner conductor and outer conductors 8. The transmission line 7 is supported by a supporting member 9 which is an insulator. In general, the dielectric block 2501 is formed integrally with and supported by the supporting member 2 using glass with a low melting point. The operation principle of the conventional dielectric resonator having the above-described construction will be described below. When the dielectric block 2501 and the coupling loop 2502 are held in the metal cavity 2401 and the transmission line 7 is connected thereto, an electromagnetic field is produced in the cavity 2401. Thus, the conventional dielectric resonator has a resonance frequency corresponding to a resonant mode. The degree of electromagnetic coupling of the dielectric resonator is a critical parameter for determining the electric characteristic of the dielectric resonator. The degree of electromagnetic coupling is determined depending on the number of lines of magnetic force across the cross section of the coupling loop 2502. That is, according to the conventional technique, the coupling loop 2502 is mechanically rotated by the handle 4c and hence the effective cross-sectional area is varied, so that the number of lines of magnetic force across the coupling loop 2502 is adjusted.
In order to match the impedance of the dielectric resonator, the electric length of the coupling loop is precisely adjusted to be an odd-integer multiple of a quarter wavelength.
However, the above-described prior art has the following drawbacks.
(1) A complicated mechanism for mechanically rotating the coupling loop is required, and hence the number of components required is increased.
(2) The means for impedance matching is limited, and the size of the coupling loop is greatly increased for lower frequencies. Also, since the coupling loop is small for higher frequencies, it is impossible to attain a higher degree of coupling.
(3) In principle, the range of frequencies in which the impedance matching can be achieved is narrow.
(4) In order to melt the glass for adhesion, a heating treatment to the dielectric member is required. The adhesive strength of glass is low, and the mechanical reliability is poor.
As a result, the following problems arise.
(1) The coupling loop is easily rotated due to vibration and impact, so that the degree of electromagnetic coupling is varied.
(2) The production process is complicated.
(3) The production cost is increased.